We investigate statistical properties of surface currents as well as coherent mesoscale eddies in the seasonally ice-covered Southern Ocean. Based on a recent regional Sea Level Anomaly satellite altimetry dataset, we compute Eddy Kinetic Energy (EKE) and detect mesoscale eddies. EKE is about one order of magnitude higher in the northern sector of the subpolar basin and over the continental slope, as compared to the middle of the subpolar gyres. An eddy detection methodology reveals that eddies are distributed evenly in the subpolar Southern Ocean, and their amplitude follows the spatial pattern of EKE. In addition to regional circulation variations, sea ice concentration arises as an important driver of eddy properties. Eddies have low amplitude and density in the pack ice, in particular in the middle of the gyres where the background circulation is unfavorable for instabilities. In contrast, the northern part of the Marginal Ice Zone is favorable for mesoscale eddies, especially cyclonic. There, eddies are stronger and their density is higher than in any other region of the ice-covered or ice-free subpolar Southern Ocean. This region is expected to be a site of frontogenesis due to sea ice melt and upwelling generated from interactions between the wind and the sea ice. While many mesoscale eddies will fall below detection level due to the small Rossby radius at high latitudes, these results contribute to understanding the interactions between mesoscale eddies, sea ice, and the background circulation in the subpolar region.

Key Points

A new satellite altimetry dataset allows the detection of mesoscale eddies in the ice-covered Southern Ocean.

Eddy generation and strength are primarily impacted by sea ice concentration and the background circulation.

While the eddy strength is damped under high sea-ice concentration, the northern edge of the sea-ice region is a hotspot of large eddies.

Plain Language Summary

Ocean eddies are spatially coherent vortices, ubiquitous in the global oceans, and are central structures contributing to shaping the global ocean circulation. In the Southern Ocean, they have been shown to play a potentially important role in the cross-front transport of heat and nutrients at mid and high latitudes. Recent improvements in satellite altimetry capabilities in the ice-covered regions allow for a first description of mesoscale eddies in the ice-covered Southern Ocean, and the investigation of their interactions with sea ice. The location of the detected eddies is consistent with the main large-scale circulation features. In addition to the background circulation, the sea ice cover has a strong impact on the size of eddies. While weaker eddies are found in the highly concentrated sea ice areas, the marginal ice zone seems to be a region with enhanced eddy activity, with more and stronger eddies detected.